The gas oil from the flash zone of a fractionator in a delayed coking process (hereinafter flash zone gas oil or “FZGO”) is a heavier product with a higher boiling point and lower quality than heavy coker gas oil. Thus, it has few uses as a refinery intermediate feedstock and would normally be used to produce heavy fuel oil, which is a low-value product. FZGO is normally recycled back as feed to the heater in a conventional delayed coking process system. This recycle, also known as a natural recycle, consumes unit capacity and thus, replaces the fresh coker feed, also known as crude vacuum residuum feed, with a vacuum residuum feed that includes recycled FZGO. Almost all delayed coking processes recycle the FZGO to extinction within the delayed coking process and thus, no external product with FZGO is produced. As a result, the conventional delayed coking process produces a lower yield of higher valued products such as, for example, gas, naphtha, light gas oil and heavy gas oil hereinafter referred to as lighter hydrocarbons. Additionally, the conventional delayed coking process produces a higher yield of low value petroleum coke.
In FIG. 1, a schematic diagram illustrates the recovery of FZGO in one embodiment of a standard delayed coking process system 100 that includes a heater 102, two coke drums 104, a fractionator 106 and a fractionator bottoms line 108. The fractionator bottoms line 108 includes vacuum residuum feed in the natural recycle that reenters the fractionator 106 with the crude vacuum residuum feed. The system 100 illustrates how a conventional delayed coking process system may be modified to remove FZGO as a separate product from the fractionator 106 for further processing or blending to produce fuel oil. Other separate products, such as gas, naphtha, light coker gas oil and heavy coker gas oil, are also removed from the fractionator 106. Although the system 100 will increase the unit capacity in the heater 102 for crude vacuum residuum feed by removing FZGO from the natural recycle, the FZGO can be difficult to process as a separate product because it contains a high asphaltene content and a high metals content. The removed FZGO thus, may adversely affect the operations and reliability of standard fixed bed catalyst hydrocracking/hydrotreating.
There are several types of hydroprocessing that can be used to upgrade crude vacuum residuum to lighter hydrocarbon products, which is referred to hereinafter as vacuum residuum hydroprocessing. Vacuum residuum hydroprocessing may include, for example, any process that converts crude vacuum residuum with hydrogen and a catalyst into lighter molecules. Vacuum residuum hydroprocessing thus, includes fixed bed catalyst hydrocracking/hydrotreating, ebullated bed hydrocracking, and dispersed catalyst hydrocracking that crack the crude vacuum residuum into hydrocarbons such as gas, naphtha, light gas oil and heavy gas oil.
In FIG. 2, a schematic diagram illustrates a vacuum residuum hydroprocessing unit 202 implemented with another embodiment of a standard delayed coking process system 200. The system 200 includes the same components as the standard delayed coking process system 100 in FIG. 1 except that the fractionator bottoms line 108 includes FZGO as part of the vacuum residuum feed in the natural recycle instead of removing FZGO as a separate product. The crude vacuum residuum enters the vacuum residuum hydroprocessing unit 202 for fixed bed catalyst hydrocracking/hydrotreating, ebullated bed hydrocracking or dispersed catalyst hydrocracking, which produces gas, naphtha, light gas oil, heavy gas oil and another source of vacuum residuum feed in feed line 204 that represents unconverted (uncracked) oil. The process illustrated in FIG. 2 suffers from the same disadvantages as the conventional delayed coking process.